Method for manufacturing tight pitch, flip chip integrated circuit packages

ABSTRACT

A flip chip packaging method to attach a die to a package substrate, the method including dipping the die into solder paste; placing the die onto the package substrate; and reflowing the solder paste to attach the die to the package substrate. Other embodiments are described and claimed.

FIELD

The present invention relates to packaging of integrated circuits, andmore particularly to flip chip integrated circuit packages.

BACKGROUND

In a flip chip packaged integrated circuit, or simply “flip chip” forshort, a die (chip) containing the integrated circuit has its activeside facing a package substrate. In the flip chip process, also formallycalled the Controlled Collapse Chip Connection (C4) evaporative bumpprocess, conductive bumps are formed and soldered to pads on the activeside. The solder bumped die is then placed face down onto matchingbonding pads on a multilayer organic package substrate. The assembly isreflowed so that the conductive bumps are soldered to pads on thepackage substrate to provide electrical connection between the activeside of the integrated circuit and the package substrate. Thiselectrical connection forms part of the so-called level 1 interconnect.The conductive bumps also provide a load bearing link between the dieand the package substrate. Usually, the conductive bumps comprisesolder.

After the die is attached to the substrate, an epoxy resin (orunderfill) is usually applied at the interface between the die and thepackage substrate to help compensate for the difference in thecoefficient of thermal expansion (CTE) between the die and the packagesubstrate, and to prevent moisture from getting to the die surface. Theflip chip may also be capped with a liquid epoxy for further protection.

FIG. 1 illustrates in simplified form a process comprising three stepsto attach and solder the conductive bumps on a flip chip die to thepackage substrate, where the steps are labeled in sequential order as“A”, “B”, and “C”. In step A, die 102 held by tool 104 is dipped intoflux resin 106. Arrow 108 pictorially represents this dipping process,where conductive bumps 110 are dipped into flux resin 106 and thenpulled out. In step B, the numeric label 112 denotes solder wettingconductive bumps 110. Arrow 114 pictorially represents die 102 beingplaced onto package substrate 116. In step C, wavy lines 118 pictoriallyrepresent heat being applied to cause reflow of the solder so thatconductive bumps 110 are soldered to pads (not shown) on packagesubstrate 116.

As integrated circuits become more complex with higher numbers of inputand output pads with a corresponding increase in the number ofconductive bumps, the pitch of the conductive bumps is expected toincrease. However, some problems that may arise with a tighter pitchusing a reflow and attach process such as that illustrated in FIG. 1are, to name just a few: opens or shorts leading to lower yield, poorwettability of the solder on the conductive bumps, and electricalconnections between conductive bumps and pads that may fail over time.Warping of the die and package substrate may increase the likelihood ofthese problems.

A low cost manufacturing process to attach a die to a package substrateresulting in good yield and reliability in a tight pitch, flip chipintegrated package is of utility.

SUMMARY

In an embodiment, a die is dipped into solder paste, placed onto apackage substrate, and then the solder paste is reflowed to attach thedie to the package substrate.

In another embodiment, to attach a die to a package substrate, solderpaste is applied to the package substrate, the die is placed onto thepackage substrate before reflow of the solder paste; and then the solderpaste is reflowed to attach the die to the package substrate.

In another embodiment, to attach a die to a package substrate, solderpaste is dispensed onto pads on the package substrate, the die is placedonto the package substrate before reflow of the solder paste, and thenthe solder paste is reflowed to attach the die to the package substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional process in flip chip manufacturing.

FIG. 2 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to an embodiment.

FIG. 3 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to another embodiment.

FIG. 4 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to another embodiment.

DESCRIPTION OF EMBODIMENTS

In the description that follows, the scope of the term “someembodiments” is not to be so limited as to mean more than oneembodiment, but rather, the scope may include one embodiment, more thanone embodiment, or perhaps all embodiments.

FIG. 2 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to an embodiment, withsub-figures in FIG. 2 labeled as “A”, “B”, “C”, and “D”, where theselabels do not necessarily imply a sequential order. For example, theprocess illustrated as “A” in FIG. 2 need not be part of an embodiment,or if it is to be included in an embodiment, then it need notnecessarily be performed before the process labeled as “B”.

In “A” of FIG. 2, a jet flux process is used whereby tool 202 applies(e.g., sprays) flux resin 204 onto package substrate 116. In “B” of FIG.2, die 102 is dipped into solder paste 206 and then removed, where arrow208 pictorially represents this dipping process. Solder paste 206 maybe, for example, a combination of flux and particulate solder. In thedipping process in “B”, the bottom portions of conductive bumps 110 areput into contact with solder paste 206 such that when tool 104 removesconductive bumps 110 from solder paste 206, there is no wetting betweenconductive bumps 110 that may lead to shorts. Sub-figure “C” of FIG. 2illustrates solder paste 206 wetting the bottom surfaces of conductivebumps 110, such that there is no bridging of solder between theconductive bumps to cause a short. Arrow 212 pictorially representsplacing die 102 onto package substrate 116.

If the process indicated in “A” of FIG. 2 is performed, then flux resin204 is already on package substrate 116 in the process of “C”, but forsome embodiments the process indicated by “A” in FIG. 2 may not beperformed so that flux resin 204 may not be present on package substrate116 in the process of “C”. For some embodiments, flux resin 204 may havea thickness of around 5 to 15 μm. The use of flux resin 204 may increasethe wettability of solder paste 206 when the process “C” in FIG. 2 isperformed.

In “D” of FIG. 2, wavy lines 118 pictorially represent applying heat soas to reflow solder paste 206 in order to solder and attach conductivebumps 110 to pads (not shown) on package substrate 116.

FIG. 3 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to another embodiment.Sub-figures in FIG. 3 are labeled as “A”, “B”, “C”, and “D”, where thealphabetical order of these labels implies a sequential order. Inprocess “A” of FIG. 3, tool 302 comprising multiple needles 304 isdipped into solder paste 206 and then removed, as pictoriallyrepresented by arrow 306. Needles 304 have the same array pattern asconductive bumps 110 on die 102.

In process “B” of FIG. 3, solder paste 206 wets the tips of needles 304,where arrow 308 pictorially represents placing the tips of needles 304close to package substrate 116 to dispense solder paste 206 onto packagesubstrate 116.

Process “C” of FIG. 3 illustrates that solder paste 206 has beendispensed onto package substrate 116 to form drops of solder paste 206at the same pitch as conductive bumps 110. Arrow 310 pictoriallyrepresents placing die 102 onto package substrate 116 so that conductivebumps 110 come into contact with corresponding drops of solder paste206.

In process “D” of FIG. 3, wavy lines 118 pictorially represent theapplication of heat to cause reflow of solder paste 206 so thatconductive bumps 110 are soldered to pads (not shown) on packagesubstrate 116.

FIG. 4 illustrates a process to attach and solder the conductive bumpson a die to pads on a package substrate according to another embodiment.Sub-figures in FIG. 4 are labeled as “A”, “B”, and “C”, where thealphabetical order of these labels implies a sequential order. Inprocess “A” of FIG. 4, tool 402 comprising multiple needles 404dispenses solder paste 206 onto pads 405 on package substrate 116.Needles 404 have small openings at their tips so that when underpressure, solder paste 206 flows through these openings so that drops ofsolder paste 206 may be dispensed onto pads 405 on package substrate116. The process of bringing tool 402 close to package substrate 116,dispensing solder paste 206, and bringing tool 402 away is pictoriallyrepresented by arrow 406. Needles 404 have the same array pattern asconductive bumps 110 on die 102.

Processes “B” and “C” in FIG. 4 are the same, respectively, as processes“C” and “D” in FIG. 3, so that part of the description for theembodiment of FIG. 3 need not be repeated with respect to FIG. 4. (Forease of illustration, pads 405 are now shown in processes “B” and “C” ofFIG. 4.)

Additional processes well known in the technology of flip chippackaging, such as applying underfill to the interface between die 102and package substrate 116 in FIGS. 2, 3, and 4, may be performed, butare not shown in these figures for ease of illustration. Also, pads,under bump metallization layers, and other protective layers, such asfor example oxide surface protectant layers, are well known but are notshown in all or some of these figures for ease of illustration.

Various modifications may be made to the described embodiments withoutdeparting from the scope of the invention as claimed below.

1. A method to attach a die to a package substrate, the methodcomprising: applying a single continuous layer of flux resin having athickness of approximately 5 to 15 micrometers to the package substrate,wherein pads are located on the package substrate, and wherein the fluxresin covers the pads on the package substrate; dipping at least aportion of the die into solder paste; placing the dipped die onto thepackage substrate after applying the flux resin, wherein the at least aportion of the dipped die is placed directly onto the applied fluxresin; and after placing the dipped die onto the package substrate toprovide electrical connection between the die and the pads on thepackage substrate, reflowing the solder paste remaining on the portionof the die to attach the die to the package substrate, wherein underfillis applied between the dipped die and the package substrate afterreflowing the solder paste.
 2. The method of claim 1, wherein the diehas conductive bumps, and wherein dipping the die into the solder pastecomprises dipping the conductive bumps into the solder paste.
 3. Themethod of claim 2, wherein reflowing the solder paste to attach the dieto the package substrate comprises: applying heat to the dipped die,wherein the heat is first applied to the dipped die subsequent toplacing the dipped die onto the package substrate; and soldering theconductive bumps to the pads.
 4. The method of claim 1, wherein the diehas conductive bumps and wherein reflowing the solder paste to attachthe die to the package substrate comprises soldering the conductivebumps to the pads.
 5. The method of claim 1, wherein the solder paste isa combination of flux and particulate solder.
 6. The method of claim 1,wherein the flux resin has a thickness in the range of 5 micrometers and15 micrometers.
 7. The method of claim 2, wherein there is no wettingbetween the conductive bumps.
 8. A method to attach a die to a packagesubstrate, the method comprising: a step for applying a singlecontinuous layer of flux resin having a thickness of approximately 15micrometers to the package substrate, wherein pads are located on thepackage substrate, and wherein the flux resin covers the pads on thepackage substrate; a step for dipping at least a portion of the die intosolder paste; and a step for placing the dipped die onto the packagesubstrate after applying the flux resin, wherein the at least a portionof the dipped die is placed directly onto the applied flux resin; and astep for reflowing the solder paste remaining on the portion of the dieto attach the die to the package substrate, wherein underfill is appliedbetween the die dipped and the package substrate after reflowing thesolder paste.
 9. The method of claim 8, wherein the die has conductivebumps, and wherein dipping the die into the solder paste comprisesdipping the conductive bumps into the solder paste.
 10. The method ofclaim 9, wherein reflowing the solder paste to attach the die to thepackage substrate comprises soldering the conductive bumps to the pads.11. The method of claim 9, wherein there is no wetting between theconductive bumps.
 12. The method of claim 9, wherein only a bottomportion of the conductive bumps is dipped into the solder paste within asolder paste container.
 13. The method of claim 8, wherein the die hasconductive bumps and wherein reflowing the solder paste to attach thedie to the package substrate comprises soldering the conductive bumps tothe pads.
 14. The method of claim 1, wherein applying the flux resin tothe package substrate comprises spraying the flux resin onto the packagesubstrate.
 15. The method of claim 1, wherein the single continuouslayer of the flux resin has a thickness of approximately 5 micrometers.16. The method of claim 8, wherein the step for applying includesspraying the flux resin onto the package substrate.
 17. The method ofclaim 16, further comprising a step for applying heat to the dipped die,wherein the heat is first applied to the dipped die subsequent to thestep for placing.
 18. The method of claim 1, wherein reflowing thesolder paste to attach the die to the package substrate comprisesapplying heat to the dipped die, wherein the heat is first applied tothe dipped die subsequent to placing the dipped die onto the packagesubstrate.
 19. The method of claim 1, wherein after applying the fluxresin to the package substrate, no additional material is deposited onthe flux resin until the dipped dies is placed onto the packagesubstrate.